We study the relation of hemopoiesis and the supporting structure of hemopoietic tissues. We attempt to obtain information concerning the nature of this relationship. In vivo, we use ultrastructural techniques and a variety of cell biological techniques to study cellular and membrane relationship between hemopoietic cells and the supporting hemopoietic stroma. Experimentally, we alter the marrow by modifying the demand for various cell type (red cells, granulocyte and platelet). We then study the mode and the rate of their delivery into the circulation and change that occur, as a result of these alteration, in marrow stroma (vascular and extravascular components). We study the receptors for various humoral factors that are required or capable of modifying hemopoiesis. We study changes in these receptors and their mode of action in marrow that is experimentally modified. In vitro, we attempt to maintain sustained hemopoiesis by growing stromal cells of bone marrow and seeding them with hemopoietic stem cell. We study the nature of their interaction in normal and experimentally modified states.